Charnockite from Baffin Island

A Deep and Meaningful Granitoid Rock

Charnockite
is a term coined by Holland (1900) to describe a type of granitic rock, usually of
a distinctive greenish-yellow hue, found at Madras, in Madras state on the southeast
coast of India (the official name of city today is Chennai, and the state is
Tamil Nadu). The colour, unusual for a granite, is due at least in part to fine
alteration and fracture fillings in the feldspars. Charnockites typify the
deep continental crust of the Earth, and they are commonly found in granulite-facies
metamorphic terranes. As with the broader family of granites,
charnockites have been ascribed diverse origins, spanning a range
of metamorphic and igneous derivations (Kilpatrick and Ellis, 1992).

The sample in the photograph is from the Pangnirtung region of
southeast Baffin Island, just north of the
Arctic Circle in Canada's eastern Arctic region, the territory of Nunavut.
It is clearly a crystalline granitic rock,
composed largely of two feldspars, perthite and andesine, but
in addition it contains roughly 5 volume percent orthopyroxene
(hypersthene). Minor minerals are quartz, myrmekite, hornblende,
biotite, magnetite and clinopyroxene, with traces of
apatite, sulphides and zircon. In strict terminology, the rock is a
hypersthene monzonite (mangerite), estimated colour index 9 percent.
Baffin charnockites are of Proterozoic age
(Pidgeon and Howie, 1975; Jackson and Morgan, 1978).

Charnockites are found on every continent but, since exposures are
limited to deep, high-grade terranes, they are unfamiliar in many regions.
Thus in the Grenville province of North America, they are well-developed
in parts of Labrador, Quebec and the Adirondacks of New York, but
uncommon in Ontario. The following literature survey of 723 articles
on charnockites hints at their worldwide occurrence.
The MINLIB
bibliography undoubtedly stresses the occurrences in the Indian
subcontinent, yet southern India may well provide the world's best
and most accessible region for the study of these interesting rocks.
They have received well-deserved attention in the past 20 years
(see, e.g., Radhakrishna, 1993; Bhattacharya et al., 2001;
Rickers et al., 2001).

RICKERS,K, MEZGER,K and RAITH,MM (2001) Evolution of the
continental crust in the Proterozoic Eastern Ghats belt, India
and new constraints for Rodinia reconstruction: implications
from Sm-Nd, Rb-Sr and Pb-Pb isotopes. Precambrian Research 112,
183-210.